On my 97 R1100 RT I seem to have a small oil leak coming out of the filler cap on the head.
Not sure if I needs another seal around the cap or if there is a rubber seal that needs replacing attached to the head that the cap sits. on any ideas

This is a common and well known issue. It is not usually the o-ring on the filler cap, but the o-ring that seals the plastic piece that the filler cap screws into. It hardens from heat cycles and no longer seals perfectly. Some have tried special viton o-rings, but I've found that the stock one lasts for 5 - 10 years, so I just change it when it starts to seep. I change both o-rings and it seals like new until it doesn't, and then I change it again.